1. Field of the Invention
The present invention relates to a charging device which charges an image bearing member with a charging roller in electrophotographic image forming apparatus and which has a cleaner cleaning the charging roller. In addition, the present invention also relates to an image forming apparatus such as copiers and printers which use the cleaning device and a process cartridge using the charging device.
2. Discussion of the Background
In conventional electrophotographic image forming apparatus, an image is typically formed by the following method:    (1) an image bearing member such as photoreceptors is charged with a charger to apply a charge having a predetermined polarity to the image bearing member (i.e., charging process);    (2) the image bearing member is exposed to imagewise light to form an electrostatic latent image on the image bearing member (i.e., light irradiation process);    (3) the electrostatic latent image is developed with a toner having a charge with the same polarity as that of the electrostatic latent image to form a toner image (i.e., developing process);    (4) the toner image is transferred to a receiving material such as papers (i.e., transferring process); and    (5) the toner image is fixed on the receiving material upon application of heat and pressure thereto to form a hard copy (i.e., fixing process).
Even after the transfer process, a small amount of toner particles remain on the surface of the image bearing member. Therefore, the surface of the image bearing member is typically cleaned by a cleaner such as cleaning blades and cleaning brushes before the next charging process to remove the residual toner particles from the image bearing member.
Recently, contact charging methods in which a voltage is applied to an image bearing member by an electroconductive charging roller which is contacted with the image bearing member or short-range charging methods in which a voltage is applied to an image bearing member by an electroconductive charging roller which is set in close vicinity to an image bearing member are typically used for the charging process. This is because these charging methods have advantages such that the amount of ozone generated due to discharging caused by the charger, and the power consumption of the charger can be reduced.
However, when residual toner particles are insufficiently removed, a problem in that the remaining toner particles adhere to the contact charging roller or the short-range charging roller occurs. This is because the remaining toner particles typically include toner particles having a charge with a polarity opposite to the polarity of the potential of the charging roller, and the reversely-charged toner particles are attracted by the charging roller, resulting in adhesion of the toner particles on the surface of the charging roller. In addition, dust such as paper dust generated by receiving papers, which has a charge with a polarity opposite to that of the potential of the charging roller is also adhered to the charging roller.
Recently a need exists for an electrophotographic image forming apparatus capable of producing high quality and high definition images. Therefore, a spherical toner having a relatively small particle diameter is typically used to form a toner image in which such small spherical toner particles can be densely adhered to an electrostatic latent image. However, such a small spherical toner has a drawback in that when toner particles remaining on an image bearing member are scraped by a cleaning blade, the toner particles often pass through the nip between the image bearing member and the cleaning blade, resulting in occurrence of bad cleaning of the surface of the image bearing member (namely, the charging roller is contaminated with toner particles). Therefore, it is necessary to clean the surface of the charging roller to prevent occurrence of various problems.
Published unexamined Japanese Patent Application No. 2002-221883 discloses a cleaning device for a charging roller in which a brush roller is brought into contact with a charging roller upon application of only its own weight thereto while being driven by the rotating charging roller. The length of the fibers serving as the hair of the brush roller is not longer than 2 mm, and preferably from 0.4 to 0.6 mm. Such a brush roller can be prepared by a method in which short fibers are planted on a roller by utilizing electrostatic force.
However, when the length of the fibers on the brush roller is short, the amount of toner particles that the brush roller can contain in the hair is limited. In a case of process cartridges including a charging roller, the charging roller has to have the same life as those of other members constituting the process cartridges, each of which has a relatively long life. Therefore, a cleaning device having such a brush roller is not suitable for such process cartridges.
In addition, it is necessary for the cleaning device to remove foreign materials such as paper dust, which adhere to the charging roller.
Because of these reasons, a need exists for a charging device with a cleaner which can efficiently clean materials electrostatically adhered to the surface of a charging roller over a long period of time.